1. Field of the Invention
The present invention relates to a magnetic levitation system for levitating and supporting a magnetizable object as a target in a predetermined position in a contactless manner through magnetic attraction or repulsion forces caused by electromagnets. More particularly, the present invention pertains to a control apparatus for controlling such a magnetic levitation system as above, which is capable of depressing a transient response presented by a magnetizable object at the beginning and the ending of levitation control.
2. Description of the Related Art
FIG. 1 illustrates a schematic diagram showing an arrangement of a target or magnetizable object 15 to be levitated, electromagnets 11 and 13, and distance detection sensors 12 and 14 in a conventional magnetic levitation system. As shown in FIG. 1, the object 15 is disposed between the electromagnets 11 and 13 and also between the distance detection sensors 12 and 14.
The object 15 is levitated by magnetic attraction and/or repulsion forces generated by the electromagnets 11 and 13. The distance detection sensors 12 and 14 detect distances from the object 15 to the sensors and generate distance signals, respectively. The generated signals are then provided to a control unit (shown in FIG. 2), which controls phases and frequencies of AC currents flowing through the electromagnets 11 and 13 in response to the distance signals, so that the object 15 is levitated at a predetermined target position between the electromagnets 11 and 13.
FIG. 2 shows a block diagram of a prior art magnetic levitation system including a control unit 20 connected to the electromagnets 11 and 13 (EMs) and the distance detection sensors (DDSs) 12 and 14. The control unit 20 comprises a sensor signal processing circuit 21, a comparator 22, a phase compensation circuit 23, a variable gain amplifier 24, a main amplifier 25 and a target position (TP) signal generator 28. The distance signals detected by the sensors 12 and 14 are inputted to the sensor signal processing circuit 21, where a current levitation position of the object 15 is calculated in response to the distance signals. The obtained current position signal is compared at the comparator 22 with a target levitation position signal from the target position signal generator 28 to generate a signal representing a displacement between the current and target positions of the object 15. The phase compensation circuit 23 determines compensation currents or compensation frequencies and phases of the AC currents flowing through the electromagnets 11 and 13 so that the displacement signal from the comparator 22 becomes zero. The electromagnets 11 and 13 receive the compensated AC currents through the variable gain amplifier 24 and the main amplifier 25, and thus provide the object 15 with adequate magnetic attraction and/or repulsion forces to levitate the object 15 at the target position. Therefore, by such a feedback control as described above, the object 15 can be levitated and supported at the target position between the electromagnets 11 and 13.
According to the aforementioned prior art control manner, a target position signal is already set at the target position signal generator 28, before a levitation control procedure is carried out. Accordingly, when the levitation system initiates a levitation control procedure of an object levitation, since a displacement between a current position and a target position is relatively large, electromagnetic forces from the electromagnets 11 and 13 to the object 15 relatively large at the starting time of the procedure. Therefore, as is represented by A in FIG. 3, when the control starts at a time t0, the objects 15 rises from a seated position C to a position above the target position B and then falls to a position under the target position B, due to feedback response characteristics etc. of the magnetic levitation system. Thus, the object 15 swings around the target position B until the levitation control becomes stable condition at t1, as shown in FIG. 3.
In other words, at the moment t0 magnetic levitation of the object 15 is started, the sensor signal processing circuit 21 outputs a current position signal indicating that the object 15 is at the seated position C, and therefore, the comparator 22 outputs a differential signal indicating a difference between the target position signal from the generator 28 and the current position signal from the sensor signal processing circuit 21. As a result, an output from the comparator 22 changes stepwise at t0 as is indicated by D in FIG. 3. Therefore, an output of the phase characteristic compensation circuit 23 also varies stepwise, which causes the object 15 to suddenly levitate to the portion above the target portion B. In response thereto, the system renders the object 15 to fall down by the feedback control, by which the object 15 fall down to the position under the target position B.
The up- and-down or fluctuation of the object 15 is gradually damped and, the position of thereof is finally stabilized at the target position B at t1.
It has been confirmed by experiments, etc., that such a fluctuation or oscillation of an object at the beginning of a levitation control procedure in a magnetic levitation system appears in such cases that a stationary position or seated position C of the object 15 is not constant with respect to a target position B, and start condition of levitation of the object 15 is not constant with respect to AC currents flowing through electromagnets 11 and 13. In the worst case, the object 15 repeatedly comes into contact with the electromagnets 11 and 13 until the object 15 is levitated at the target position B in a stable manner. As a result thereof, the object 15 and/or the magnets 11 and 13 could be damaged and dust could be produced thereby. Such dust causes problems in particular when the magnetic levitation system is utilized in a semiconductor production system and so on.
Further, when levitation control procedure is stopped, the target position signal from the circuit 28 is suddenly lowered to indicate the seated position C, while the current position signal from the sensor signal processing circuit 21 is still indicate the target position B. As a result, the output from the comparator 22 and hence the output from the compensation circuit 23 change stepwise as shown by E in FIG. 4, when the control procedure is stopped at t2, the object 15 is steeply descending to the seated position C and it rebounds therefrom, resulting in damage to the object 15 and/or the magnets 11 and 13 and the production of dust.
The present invention has been accomplished in order to obviate the aforementioned problems of the prior arts. Thus, it is an object of the present invention to provide a control apparatus for controlling a magnetic levitation system, which is capable of preventing fluctuation or oscillation of a targeted object from occurring at the beginning and ending of a levitation control procedure, and thereby of preventing the object from coming into contact with electromagnets due to the oscillation.
In view of one aspect of the present invention, it provides a control apparatus in a magnetic levitation system, for controlling attraction and/or repulsion forces created from a pair of electromagnets to levitate an object therebetween at a predetermined target position in a contactless manner, in response to a displacement of the object from the target position, the control apparatus comprising: (a) a start/stop detector for detecting a start and stop of a levitation control procedure and generating a control signal having a predetermined time duration when either of the start and stop of the levitation control procedure is detected; (b) a compensation circuit for providing a compensation signal to compensate AC currents flowing through the electromagnet so that the displacement of the object from the target position becomes zero; (c) an integrator for integrating the compensation signal; and (d) a switching circuit for providing the compensation signal outputted from the compensation circuit when the control signal is not generated from the start/stop detector, and the integrated compensation signal outputted from the integrator when the control signal is generated, whereby the magnetic forces from the electromagnets to the object gradually varies at the beginning and ending of the procedure and hence the object is gradually levitated and seated.
In view of another aspect of the present invention, it provides a control apparatus in a magnetic levitation system, for controlling attraction and/or repulsion forces created from a pair of electromagnets to levitate an object therebetween at a predetermined target position in a contactless manner, in response to a displacement of the object from the target position, the control apparatus comprising: (a) a start/stop detector for detecting a start and stop of a levitation control procedure and generating a control signal having a predetermined time duration when either of the start and stop of the levitation control procedure is detected; (b) a compensation circuit for providing a compensation signal to compensate AC currents flowing through the electromagnet so that the displacement of the object from the target position becomes zero; (c) an integrator for integrating a signal representing the displacement of the object from the target position; and (d) a switching circuit for providing the displacement signal to the compensation circuit when the control signal is not generated from the start/stop detector, and the integrated displacement signal outputted from the integrator to the compensation circuit when the control signal is generated, whereby the magnetic forces from the electromagnets to the object gradually varies at the beginning and ending of the procedure and hence the object is gradually levitated and seated.